1. Field of the Invention
The present invention relates to an optical head used for data-writing or data-reading with an optical storage medium. It also relates to a method of making such an optical head.
2. Description of the Related Art
To improve the data storage density of a magneto-optical disk, several methods have been proposed in the past. One of them is to increase the numerical aperture (NA) of the objective lens. The use of a lens having a higher NA makes it possible to produce a smaller laser spot on the recording layer of the storage disk, and hence an improved data storage density will result. The increase of the NA can be achieved by using two or more lenses in combination. These lenses may be attached to a lens holder of the optical head. As known in the art, coma, caused by the tilt of the storage disk, will become greater in proportion to the third power of the NA, while spherical aberration, caused by unevenness of the disk thickness, will increase in proportion to the fourth power of the NA. Thus, without taking any precaution, these aberrations may become unacceptably larger as the NA increases. Further, it is necessary to use a surface-recording type magnetic modulation coil in order to increase the data transfer speed and also to improve the data storage density.
In light of the above, the front illumination type optical heads have often been employed. In this system, the transparent substrate of the disk does not come between the recording layer of the disk and the optical head, and instead the recording layer is held in direct facing relation to the optical head. To improve the NA of the lens system in the front illumination type, the optical head includes a lens holder to which a combination of two lenses is attached. The lens holder may also carry a coil needed for generating a magnetic field. Such an optical head is often designed as a slider which floats slightly over the storage disk in rotation by the action of the air drawn between the optical head and the magneto-optical disk. Thus, the above coil can be brought very close (in the micron or submicron order) to the storage disk. Accordingly, the data storage density and data transfer speed can both be improved.
Unfavorably, the above-described conventional optical head has the following drawbacks.
When the two lenses are not positioned accurately to the lens holder, come will occur. As noted above, coma will become greater as the NA is rendered higher. Thus, considerable care should be taken in installing the lenses on the lens holder. In particular, the lens near the storage disk should be positioned with greater accuracy than the farther lens.
In the prior art, however, no particular precaution to prevent the tilting of the lens has been taken. Accordingly, it is difficult to achieve efficient fabrication of high-quality optical heads.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide high-quality optical heads in which the occurrence of coma is eliminated or rendered less conspicuous than in the prior art.
Another object of the present invention is to propose an efficient method of making such advantageous optical heads.
According to a first aspect of the present invention, an optical head is provided which includes a lens holder, first and second lenses, a coil and a transparent plate. Specifically, the lens holder is formed with a throughhole and includes an obverse surface and a reverse surface opposite to the obverse surface. The obverse surface faces the optical data storage medium to be used. The first lens is supported by the lens holder. The second lens is arranged in the through-hole in a manner such that the second lens is closer to the storage medium than the first lens is. The coil generates a magnetic field. The transparent plate is bonded to the lens holder. The transparent plate includes a first surface on which the coil is provided and a second surface opposite to the first surface. The second lens is bonded to the second surface of the transparent plate.
With the above arrangement, it is easy to position the second lens accurately by depending upon the transparent plate. Accordingly, the occurrence of coma can be restrained to a great extent. Further, since the positioning of the second lens is performed simply by attaching the lens to the transparent plate, the fabrication efficiency of the optical head is advantageously improved.
Preferably, the transparent plate may entirely cover the obverse surface of the lens holder.
Preferably, the second lens may include a flat lens surface and a non-flat lens surface, wherein the flat lens surface is held in surface contact with the transparent plate.
Preferably, the second lens may be provided with a flange to be held in engagement with the through-hole of the lens holder. This arrangement ensures easy positioning of the second lens within the through-hole.
Preferably, the first lens may be attached to the reverse surface of the lens holder. In this manner, the positioning of the first lens can be easily performed depending upon the lens holder.
Preferably, the lens holder may be made of an electrically non-conductive material. To ensure sufficient rigidity, the lens holder is made of ceramics for example. However, some ceramic materials are electrically conductive, while others are not. If the lens holder is made of such a conductive material, the holder may need to be coated by a non-conductive layer for insulation from other conductive components (e.g. a lead for the magnetic field-generating coil). In this case, however, when the coil is powered, the lens holder may unfavorably have some capacitance, depending upon the forming condition of the insulating layer. In light of this, preferably the lens holder is made of a nonconductive material (alumina for example). Then, no insulating layer as noted above needs to be made on the lens holder. This is advantageous to improving the fabrication efficiency and providing excellent impedance characteristics.
Preferably, the second lens and the transparent plate may have equal or substantially equal refractivity, so that light is not reflected as proceeding from the second lens into the transparent plate. In this case, the assembly of the second lens and the transparent plate can be treated as a single lens that has a prescribed thickness (=[the second lens thickness]+[the transparent plate thickness]). This implies that the thickness of the second lens is made smaller when the transparent plate is provided. The amount of lens thickness reduction is proportional to the thickness of the transparent plate, so that the total thickness of the two components is constant. In other words, the second lens and the transparent plate are complementary in thickness.
Preferably, each of the first and the second lenses may be an achromatic lens. This is advantageous to coping well with the shift in wavelength of the light beam.
Preferably, the lens holder may be supported by an elastic suspension so as to float above the storage medium when the storage medium rotates.
According to a second aspect of the present invention, an optical disk apparatus is provided which includes a data-recording optical disk, a first optical head and a second optical head. Specifically, the optical disk includes a first surface and a second surface opposite to the first surface. The first optical head faces the first surface of the disk. The second optical head faces the second surface of the disk and is arranged symmetrically to the first optical head with respect to the optical disk. Each of the first and the second optical heads includes: a lens holder formed with a through-hole; a plano-convex lens arranged in the through-hole and including a flat lens surface; and a transparent plate bonded to the lens holder. The transparent plate is held in surface contact with the flat lens surface.
Preferably, the first and the second optical heads may be urged toward the optical disk with an equal or substantially equal force, so that the disk between the two optical heads will not warp or deform in any unfavorable manner.
According to a third aspect of the present invention, a method of making an optical head is provided which includes the following steps.
To begin with, a transparent first plate is prepared. Then, a plurality of coils are formed on the first plate. To cover the coils, an insulating protection layer is formed. Then, a second plate formed with a plurality of through-holes is prepared. By bonding the first and the second plates to each other, a plate assembly is produced. Then, a lens is placed in each of the through-holes. At this step, the lens is bonded to the first plate for accurate positioning.
Preferably, the method may further include the step of cutting the plate assembly in a manner such that each of the through-holes is kept undivided.
Preferably, the method of the present invention may further include the step of reducing a thickness of the first plate after the protection layer forming step and before the plate assembly producing step.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.